A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, in the range of 5 to 20 nm, in particular around 13 nm.
Such radiation is termed extreme ultra violet (EUV) or soft X-ray and possible sources include, for example, laser produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. These types of radiation require that the beam path in the apparatus be evacuated to avoid beam scatter and absorption. Because there is no known material suitable for making a refractive optical element for EUV radiation, EUV lithographic apparatus must use mirrors in the radiation (illumination) and projection systems. Even multilayer mirrors for EUV radiation have relatively low reflectivities and are highly susceptible to contamination, which may further reduce their reflectivity and hence the throughput of the apparatus. This may impose further specifications on the vacuum level to be maintained and may necessitate especially that hydrocarbon partial pressures be kept very low.
In a typical discharge plasma source, plasma is formed by an electrical discharge. The plasma may then be caused to compress so that it becomes highly ionized and reaches a very high temperature, causing the emission of EUV radiation. The material used to produce the EUV radiation is typically xenon gas or lithium vapor, although other gases or vapors such as krypton gas or tin or water vapor may also be used. However, these gases or vapors may have a relatively high absorption of radiation within the EUV range and/or be damaging to optics further downstream of the projection beam and their presence should therefore be minimized in the remainder of the lithographic apparatus. A discharge plasma source is disclosed, for example, in U.S. Pat. No. 5,023,897 and U.S. Pat. No. 5,504,795, both of which are incorporated herein by reference.
In a laser produced plasma source a jet of, for example, (clustered) xenon may be ejected from a nozzle. At some distance from the nozzle, the jet is irradiated with a laser pulse of a suitable wavelength for creating a plasma that subsequently will radiate EUV radiation. Other materials, such as water droplets, ice particles, lithium or tin vapor, etc. may also be ejected from a nozzle and be used for EUV generation. In an alternative laser-produced plasma source, a solid (or liquid) material is irradiated to create a plasma for EUV radiation. Laser produced plasma sources are, for example, disclosed in U.S. Pat. No. 5,459,771, U.S. Pat. No. 4,872,189, and U.S. Pat. No. 5,577,092, all of which are incorporated herein by reference.
A common feature of the above sources is that their operation induces a background pressure of some source gas or gases (also including vapors) in or near the source region. Source gases include those gases or vapors of which the plasma is to be created for EUV generation, but also gases or vapors produced during source operation by, for example, laser irradiation of a solid or liquid material. The source gases should be confined to the source region since they may be a cause of substantial absorption of EUV radiation or be a cause of contamination and damage in the remainder of the lithographic apparatus. The particles present in the source gases are hereinafter also referred to as debris particles.
International Patent Application Publication No. WO 99/42904, incorporated herein by reference, describes a filter including a plurality of foils, often referred to as a so-called “foil trap”, which in operation is oriented such that radiation propagates along the foils and, as such, through the filter, while debris particles moving off the propagation direction of the radiation are trapped by the foils. A buffer gas is supplied to cool the debris particles and, as such, to further enhance the probability that the debris particles are trapped by the foils.
European Patent Application Publication No. 1 329 772 A2, also incorporated herein by reference, describes the use of a buffer zone, separated from a source zone in which the radiation is generated. A wall is positioned between the source zone and the buffer zone. The wall has a beam aperture for propagation of the radiation from the source zone to the buffer zone. Buffer gas is supplied to the buffer zone and buffer gas is removed from the buffer zone such that the pressure in the buffer zone is less than or approximately equal to the pressure in the source zone, so as to prevent buffer gas to flow to the source zone, and, as such, to prevent the occurrence of an increase of the pressure in the source zone.
International Patent Application Publication No. WO 03/034153, incorporated herein by reference, describes the use of a so-called contaminant trap, including two sets of channels arranged radially about the optical axis and in series with respect to each other. Buffer gas is supplied to a space between the two sets. A part of the gas flows from this space to a radiation entrance of the first set of channels and another part of the gas flows through a radiation exit of the second set of channels.